Spectral analyzer and direction indicator for laser guided aircraft landing system

ABSTRACT

A spectral analyzer and direction indicator system (10) is disclosed and includes optical channels (20,30,40) for providing detected optical information indicative of incidence direction and spectral content of incident radiation. Each optical channel includes a reflector element (11,17,23) having a non-ruled section (11a, 17a, 23a) and a spectrally dispersing ruled section (11b, 17b, 23b); an analytical optical system (13,19,25); and a detector array (15,21,27). For each reflector element the non-ruled section is tilted in one or two directions relative to the ruled section. The disclosed spectral analyzer and direction indicator system is advantageously utilized as an aircraft based sensor in an aircraft landing system having ground based lasers (29L, 29R, 33L, 33R, 37L, 37R) directed into the landing approach path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The disclosed invention generally relates to apparatus for detectingessentially collimated radiation, measuring wavelength values withinthat radiation, determining the direction to the source of thatradiation within an extended field-of-view, and observes the coherencelength of sources so as to discriminate between essentially coherent(e.g., laser) and non-coherent radiation, all of which is accomplishedsimultaneously and in staring fashion (i.e., without scanning).

2. Background Art

The prior art includes a variety of devices for detecting coherentradiation. Examples of such prior art devices are discussed below.

U.S. Pat. No. 3,824,018, issued to Crane, Jr., discloses an unequal pathinterferometer. The interferometer scans so as to cause a change in thedifference in the two optical paths which will modulate the intensity ofthe recombined coherent radiation by varying interference effects.

U.S. Pat. No. 4,147,424, issued to Foster, et al., discloses a systemfor detecting the presence of coherent radiation having an interferencefilter with a pair of partially reflective mirrors, with the spacingbetween the mirrors being varied sinusoidally to change the transmissionwavelength of the mirrors. The output is compared to an establishedreference signal to produce an identifying indication of the detectedradiation.

U.S. Pat. No. 4,170,416 discloses apparatus for detecting the presenceof coherent radiation in the presence of incoherent ambient radiation,and for determining the intensity, the wavelength, or the thresholddirection of such coherent radiation. The apparatus includes a FabryPerot etalon having three regions of different thickness.

U.S. Pat. No. 4,183,669, issued to Doyle, and U.S. Pat. No. 4,185,919,issued to Williamson et al., disclose a quadrant detection system usingan objective lens and a holographic lens. The holographic lens has lenselements in four quadrants with each quadrant having a focal pointcorresponding to the position of adjacent photoelectric detectors.

U.S. Pat. No. 3,858,201, issued to Foster, discloses a system fordetermining a direction from which an illuminating laser beam isreceived. The system includes a cylindrical optical system for focusinga laser beam as a sharp line image.

U.S. Pat. No. 4,309,108, issued to Siebert, discloses an analyzer forcoherent radiation for discriminating wavelength from a single pulse orfrom a continuous wave radiation and to determine the relative angularposition of the source of the radiation. The analyzer includes at leastthree unequal length path interferometers and detectors for detectingthe radiation transmitted through the interferometers.

The foregoing prior art devices are generally complex, relying to alarge extent on classical electrooptical techniques and/or the use ofscanning.

While the prior art devices exhibit various combinations of features formeasuring characteristics of radiation, none, however, has theversatility or all of the particular and extensive features of thedisclosed invention combined into a single instrument without the needto scan.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to provide a spectralanalyzer and direction indicator that without resorting to scanningdetects essentially collimated radiation, and with respect to thatradiation, simultaneously measures quantities that uniquely determine(1) its wavelength values within an extended spectral band, (2) itsdirection within an extended field-of-view, and (3) its coherence lengthso as to discriminate between radiation that is coherent or essentiallycoherent and non-coherent. The principle of operation for the disclosedinvention applies equally well in any spectral region (e.g., fromultraviolet to microwave) for which there exist suitable dispersive anddetecting elements.

It is also an object of the present invention to provide an efficientspectral analyzer and direction indicator responsive to essentiallycollimated radiation.

Another object of the invention is to provide a rugged and versatilespectral analyzer and direction indicator responsive to essentiallycollimated radiation.

Still another object of the invention is to provide a spectral analyzerand direction indicator responsive to radiation that is eitheressentially coherent (e.g., laser) or non-coherent.

A further object of the invention is to provide a non-scanning spectralanalyzer and direction indicator that detects essentially collimatedradiation, discriminates between radiation that is essentially coherent(e.g. laser) and non-coherent (i.e., spectrally broad), and that locatesthe direction within a specified but extended field-of-view from whichthat radiation is received.

An additional object of this invention is to provide a staring spectralanalyzer and direction indicator that measures wavelength valuescontained within the detected radiation, which can be either coherent ornon-coherent, and that can occur anywhere within a specified butextended spectral interval.

Still a further object of the invention is to provide a spectralanalyzer and direction indicator responsive to collimated or essentiallycollimated radiation and capable of discriminating a plurality ofsources.

A still further object of the invention is to provide a spectralanalyzer and direction indicator that is rugged enough to be readilyutilized in vehicles.

Another object of the invention is to provide a spectral analyzer anddirection indicator which has reduced internal field of viewrequirements.

Still another object of the invention is to provide an aircraft landingsystem which utilizes ground based lasers as reference beams for anaircraft-based spectral analyzer and direction indicator.

The foregoing and other objects of the invention are accomplished in aspectral analyzer and direction indicator system which includes one ormore optical channels wherein each channel includes a reflector elementhaving two separate sections for respectively providing diffracted andnon-diffracted optical information, and each channel further includes ananalytical optical system for appropriately focusing the opticalinformation and detector apparatus for detecting the opticalinformation. The detected information is utilized to determine spectralcontent and incidence direction of incident radiation. In oneembodiment, the reflector element sections are angled or tilted relativeto each other so as to intersect along a first line. In a furtherembodiment, one or both of the reflector element sections are furthertilted about a line perpendicular to the first line.

In the aircraft landing system, ground based lasers adjacent a runwayare appropriately aimed for detection by an aircraft-based spectralanalyzer and direction indicator system, such as the system disclosedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the disclosed invention will readily beappreciated by persons skilled in the art from the following detaileddescription when read in conjunction with the drawing wherein:

FIG. 1 is a top plan schematic view illustrating the disclosed spectralanalyzer and direction indicator system.

FIG. 2 is an elevational schematic view illustrating an optical channelof the disclosed spectral analyzer and direction indicator system ofFIG. 1.

FIG. 3 is a schematic of a detector array as utilized in the spectralanalyzer and direction indicator of FIG. 1, and illustrates relativelocations of the optical information focused, on such detector array.

FIG. 4 is a perspective view of a further embodiment of an opticalchannel for use in the disclosed spectral analyzer and directionindicator system.

FIG. 5 is a schematic of a detector array as used in the optical channelof FIG. 4 and illustrates relative locations of the optical informationfocused on such detector array.

FIG. 6 is a top plan view illustrating the locations of a pair of lasersadjacent a runway in accordance with the aircraft landing system of theinvention.

FIG. 7 is a top plan view illustrating the locations of a pair of lasersadjacent a runway in accordance with the aircraft landing system of theinvention.

FIG. 8 is a side elevation view illustrating the elevation directions ofthe laser beams provided by the lasers shown in FIG. 6.

FIGS. 9a through 9e illustrate a sequence of graphical displays providedto an aircraft pilot by the disclosed aircraft landing system.

DETAILED DESCRIPTION

In the following detailed description and in the several figures of thedrawing, like elements are identified with like reference numerals.

While the disclosed invention is useful with different wavelengths ofessentially collimated radiation (e.g., from ultraviolet to microwave),the disclosed systems will be primarily described relative to spectrallynarrow (e.g., laser) radiation and for wavelengths typical of thevisible and infrared regions. This will facilitate the understanding ofthe principles involved which can then be applied to other sources ofradiation.

For reference purposes, the following spectral analyzer and directionindicator systems are discussed relative to a three dimensionalright-handed Cartesian coordinate system and directional angles measuredrelative to the Cartesian coordinate system. In FIG. 1, which is a topplan view, the x and y axes are shown while the z-axis is understood tobe orthogonal to the x and y axes and having a positive direction out ofthe plane of the figure. In the elevational view of FIG. 2, the y and zaxes are shown, while the x-axis is understood to be orthogonal. In theperspective view of FIG. 4, the x, y and z axes are shown.

As references for incidence direction, the angles θx and θy, areprovided, whereby θx is measured in the xz plane, and θy is measured inthe yz plane. FIG. 2 illustrates the measurement of θy relative to thez-axis and FIG. 4 illustrates the measurement of θx relative to thez-axis.

The z-axis can be considered as being along the center line (line ofsight) of the external field of view (FOV), which refers to the field ofview over which a spectral analyzer and direction indicator can receiveincident radiation. In other words, external FOV refers to the sectionof space which a spectral analyzer and direction indicator can observe.Internal field of view (FOV) in the disclosure generally refers to thefield of view of one or more of the optical components within a spectralanalyzer and direction indicator. Such optical components includereflectors, lenses, and detector arrays.

The spectral analyzer and direction indicator system 10 shown in the topplan view of FIG. 1 includes a first reflector element 11 having anon-ruled section 11a and a ruled section 11b, wherein the rulings areparallel to the x-axis. An analytical optical system 13, shown as asimple lens, focuses the essentially collimated reflected radiation onto a linear detector array 15. The detector array 15 has parallelelongated detector elements which are parallel to the x-axis. Theelongated elements in detector arrays 15 and 21 in FIG. 1 are arrangedparallel to the plane of the figure and thus resolve relative to adirection that is out of the plane of the figure but not necessarilyalong the z-axis. If the elements are segmented, some resolution will beobtained along the x-axis for the detector array 15, and along they-axis for the detector array 21.

In FIG. 1, the reflector element 11, the analytical optical system 13,and the detector array 15 will be referred to herein as an opticalchannel 20. As will be discussed more fully below, the position of thenondiffracted reflected image on detector array 15 for optical channel20 (nominally parallel to the y-axis) is indicative of the θy direction.The position of the diffracted image detected by the detector array 15along the y-axis is indicative of the angle of diffraction resultingfrom the ruled reflector section 11b. The positions of both thediffracted and undiffracted images on the detector array 15 along thex-axis represent the θx component of the incidence direction. This θxcomponent will be detected in this optical channel but will be resolvedonly to the extent permitted by any segmentation of the detectorelements.

In FIG. 1, the reflector element 17, the associated analytical opticalsystem 19, and the detector array 21 are referred to herein as anoptical channel 30. The spectral analyzer and direction indicator 10 ofFIG. 1 further includes a second reflector element 17, which has anon-ruled section 17a and a ruled section 17b wherein the rulings areparallel to the y-axis. A second analytical optical system 19, shown asa simple lens, focuses the reflected radiation on to a linear detectorarray 21. The detector array 21 has parallel elongated detector elementswhich are parallel to the y-axis. The position of the undiffracted imagedetected by the detector array 21 along the x-axis is indicative of theθx direction of the incident radiation, as well as the angle ofincidence relative to the ruled reflector section 17b. The position ofthe diffracted image detected by the detector array 21 along the x-axisis indicative of the angle of diffraction resulting from the ruledsection 17b. The positions of both the diffracted and undiffractedimages on the detector array 21 for the optical channel 30 (nominallyparallel to the y-axis) represent the θy component of the incidencedirection. This θy component will be detected in this optical channel,but it will be resolved only to the extent permitted by any segmentationof the detector elements.

The component of the angle of incidence parallel to the rulings on ruledsection 11b or 17b is not diffracted. This component of the incidenceangle is affected in the same manner as it would be if the ruledsections were unruled (i.e., a plane mirror). Since the disclosedspectral analyzer and direction indicator is a two-axis system, bothanalytical optical channels and their associated detector arrays musthave sufficient angular subtense (i.e., internal FOV) to accommodate allangular and linear excursions of interest in both axes.

As indicated in FIG. 1, the optical channels are at right angles to eachother. Such orientation allows for a straightforward determination ofthe incidence direction angles θx and θy.

Referring to the elevational view of FIG. 2, shown therein is theoptical channel 20. As shown, the relative to each other so that theirrespective planes intersect along a line parallel to the x-axis andparallel to the rulings on the ruled reflector section 11b. As measuredfrom the incident surfaces of the reflector sections 11a and 11b, theangle formed is less than 180 degrees. The angled relationship betweenthe reflector sections 11a and 11b is such that no interchange ofrelative position of the two images occurs. The angled relationshipfurther results in reduced requirements for internal FOV in theanalytical optical system 13 and for angular subtense in the detectorarray 15 in the direction of change indicated by θy, which lies in they-z plane of diffraction from the ruled reflector section 11b.

The elongated detector elements in detector array 15 of FIG. 2 arearranged at right angles to the plane of the figure. Thus, they resolvein a direction along the length of the array as shown in the figure. Ifthe detector elements are segmented, some resolution will be obtained inthe direction at right angles to the plane of the figure.

The optical channel 30, including the reflector 17, the analyticaloptical system 19, and the detector array 21, is arranged similarly tothe optical channel shown in FIG. 2. The angled relationship between thenon-ruled section 17a and the ruled section 17b results in reducedrequirements for internal FOV in the analytical optical system 19 andfor angular subtense in the detector array 21 in the direction of changeindicated by θx, which lies in the x-z plane of diffraction from theruled reflector section 17b.

The relative areas of the ruled and unruled sections of reflectors 11and 17 can be adjusted to correct for different reflectances. Forexample, if the reflective efficiency of the ruled section (e.g., adiffraction grating) 11b is less than that of the non-ruled section(e.g., a plane mirror), then the area of the ruled section can be madecorrespondingly larger so as to intercept a greater portion of theincident energy. The relative areas of the these sections can be made,for example, inversely proportional to reflectance. In this manner, thetotal radiated power incident on the reflectors is shared in such a waythat the resulting signal to noise ratios for the two images will be aswell matched as possible.

Referring now to FIG. 3, shown therein is a schematic of the incidentside of the detector array 15 shown in FIGS. 1 and 2. The D axisidentifies the direction of diffraction caused by the ruled reflectorsection 11b. The D axis also identifies the direction of change relativeto changes in the incidence direction θy. Displacements along the P axisrepresent the component of the angle of incidence at right angles tothat resolved by the particular optical channel (i.e., parallel to therulings on the ruled section). The two optical channels in combinationresolve both components of the angle of incidence, but each channel musthave sufficient angular subtense (i.e., internal FOV) along bothcoordinate axes to detect all image positions of interest.

The detector array 21 is substantially similar to the detector array 15,but because optical channel 20 and optical channel 30 are arranged atright angles to each other, the elongated elements of array 21 areparallel to the y-axis. The detector array 21 detects and resolves imageposition in the direction of diffraction caused by the ruled reflectorsection 17b, and also in the direction of change in the incidencedirection θx.

As is well known, the ruled reflector sections 11b and 17b will reflectincident radiation at angles which depend on both spectral content andincidence direction. The following will describe the determination ofspectral content for information from the ruled reflector section 11b,and is also applicable to the ruled reflector section 17b.

Rays for both incidence angle I and diffraction angle D are measuredrelative to the normal to the ruled reflector sections 11b or 17b. Theconvention for both of these angles is that rays on opposite sides ofthe normal have opposite algebraic signs. The grating equation belowrelates one component of the angle of incidence (the component normal tothe rulings) to the angle of diffraction: ##EQU1## The symbols representthe following values:

TABLE I

I: angle of incidence

D: angle of diffraction

W: wavelength

n: order of diffraction

d: spacing of adjacent rulings on ruled reflector section

Solving Equation 1 for wavelength W provides the following: ##EQU2## Thewavelengths of the spectral component of the incident radiation are,therefore, determined by the detector positions of the non-diffractedand diffracted images. The location of the diffracted image on thelinear detector array 15 will depend on several characteristics of theincident radiation. In addition to the dependence on wavelength anddirection as described above, to the extent that the incident radiationis collimated, the radiation diffracted from the grating also will becollimated. For a spectrally narrow source (e.g., a laser), a focusedspot is produced on the detector array 15. The location of the focusedspot on the detector array provides a measure of the angle ofdiffraction D.

For a point source that is spectrally broad, the diffracted image on thedetector array 15 will not be a single focused spot but will be an imagethat occupies an angular interval along the array whose extent isdetermined by the spectral content of the source. Each illuminateddetector element subtends and corresponds to a small range of angles ofdiffraction from the associated grating, and the output level of thedetector element represents the amplitude of a portion of the spectrum.Thus, both the spectral signature of the point source and its angularposition within the FOV can be determined when the information providedby the illuminated detector elements is suitably interpreted.

An expression for coherence length L is: ##EQU3## where ΔW is thespectral bandwidth in the radiation. Since the spectral analyzer anddirection indicator can yield values for both wavelength and spectralbandwidth, coherence length can be obtained from measured quantities towithin an upper limit set by the spectral resolution of any particularinstrument.

The location of the image from the non-ruled section 11a (undiffractedimage) on detector array 15 will be a function of both coordinatecomponents of the angle of incidence. The θy component will be detectedand resolved. The θx component will be detected, but will be resolvedonly to the extent permitted by any segmentation of the elongateddetector elements. Therefore, the location of the non-undiffracted imageprovides information as to the angle of incidence I, since the smoothreflector portion 11a remains at a known fixed angle relative to theruled reflector section 11b.

The detected locations on the detector array 15 of the non-diffractedand diffracted images are appropriately processed for determiningwavelength W in accordance with Equation 2. Also, the location of thedetected non-diffracted image is indicative of the incidence directionθy.

The foregoing analysis is also utilized with respect to thenon-diffracted and diffracted images detected on the detector array 21.The locations of such images are appropriately processed to determinewavelength W and the incidence direction θx.

The performance of the optical channel 20 can best be understood furtherwith reference to the images focused on to the detector array 15. Shownin FIG. 3 are two spots corresponding to the non-diffracted anddiffracted images of a spectrally narrow source (e.g., a laser) observedby the optical channel 20. The two spots define a line which is parallelto the D axis which identifies the direction of diffraction. For allsuch observed sources, the two spots will always form a line parallel tothe D axis. This line will move in response to changes in the θxincidence angle. A similar line of two images on detector array 21 willmove parallel to itself in response to changes in the θy axis. Thedistance separating the spots will be a function of both wavelength andthe incidence direction θy. In order to avoid confusion, the includedangle between the reflector sections 11a and 11b is appropriatelydetermined so that the relative positions of the two images will neverinterchange. Thus, the two spots will always be identifiable for thespectral range of interest and all directions within the external FOV.Specifically, the minimum distance between the non-diffracted anddiffracted spots must always be at least three detector elements alongthe D axis. Thus, there is a lower limit for the internal FOV that canbe achieved for the optical channels 20 and 30, and accordingly so forthe detector arrays 15 and 21.

Referring now to FIG. 4, shown therein is a modification of the opticalchannel 20 shown in FIG. 2. Specifically, shown in FIG. 4 is an opticalchannel 40 oriented similarly to the optical channel 20 and whichincludes a reflector element 23, an analytical optical system 25, shownas a simple disk, and a detector array 27. The reflector element 23includes a smooth non-diffracting section 23a and a ruled reflectorsection 23b, wherein the rulings are parallel to the x-axis. As shown,the smooth reflector section 23a is tilted along two angular axesrelative to the ruled diffracting section 23b. First, the reflectorsection 23a is tilted about an axis c which is parallel to the rulingson the ruled reflector section 23b. That tilt provides an included angleφ1 which is measured in a plane parallel to the yz plane. The reflectorsection 23a is further tilted about an axis d which is perpendicular tothe axis c and is in the plane of the reflector section 23a. Therotation about the d-axis is at an angle φ2 which is measured in a planeparallel to the xz plane.

The optical channel 40 achieves reduced requirements for internal FOV inthe analytical optical system 25 and for angular subtense in thedetector array 27 along one axis. FIG. 5 is a schematic representationof the incident side of the detector array 27. The angular subtense asillustrated in FIG. 5 is reduced in a direction parallel to the D axis.By way of example, for a spectrally narrow source (e.g., a laser) twospots will be imaged on the array 27, and the diffracted spot willalways be imaged to the right of the non-diffracted spot. The verticallines through the spots in FIG. 5 illustrate typical paths along whichsimultaneous displacements parallel to the D axis will occur. Fordisplacements parallel to the P axis, the horizontal separation betweenthe spots will remain essentially the same. Therefore, there is norequirement that the spot positions never overlap in a directionparallel to the D axis. To assure unambiguous identification of thespots under these operating conditions, the elements of the detectorarray must be segmented in a direction parallel to the P axis. Minimumseparation of the spots in the P direction must be three detectorsegments. Representative segmentation of the otherwise elongatedelements is illustrated in FIG. 5.

As a trade-off for reducing the internal FOV along the D axis, which isthe largest, the internal FOV along the P axis must be increased becauseof the requirement of separating the spots along P axis. However, it hasbeen determined that substantial optical advantage for the opticalsystem 25 results from reducing the largest internal FOV, which is alongthe D axis.

For a two-axis spectral analyzer and direction indicator system, anadditional optical channel similar to the optical channel 40 would beutilized at right angles to the optical channel 40.

The foregoing described spectral analyzer and direction indicator canobserve both direction and wavelength of a radiation source in staringfashion over a substantial internal FOV (e.g., ten (10) to twenty (20)degrees) and spectral bandpass (e.g., one octave). The external FOV ofany optical system in which a spectral analyzer and direction indicatoris placed will be determined independently by any foregoing optics. Suchoptics must provide the essentially collimated radiation required by thereflecting sections (e.g., 11a and 11b in FIG. 1). Such external optics,for example, might consist of a telescope, in which case the externalFOV would be less than the internal. In exchange for this reduced FOV,however, considerable light gathering ability is gained. The reflectingsections themselves can function as the collecting optical elements forthe remainder of the system, however, in which case the internal andexternal FOVs would be identical.

The disclosed spectral analyzer and direction indicator isadvantageously utilized as an aircraft based sensor of an aircraftlanding system which further includes ground based laser sources.

FIG. 6 illustrates in plan view a pair of lasers 29L and 29R whichradiate respective beams 31L and 31R into the landing approach flightpath. Upon entering the region of overlap of the beams 31L and 31R, anaircraft with a spectral analyzer and direction indicator, such asdisclosed above, will have on-board information instantaneouslyindicative of position relative to the runway.

Specifically, the following functions can be performed within theaircraft: (1) verification of the laser signals as being the propersources; (2) determination of relative heading in both azimuth andelevation with respect to the aircraft's own coordinate system; and (3)determination of range to the runway.

Verification of the signals consists of assuring that the sources arelasers of the proper wavelength and not sources that are bright andspectrally broad that also contain the laser wavelength. A narrowbandpass filter can be introduced that spectrally is slightly wider thanthe bandwidth of the laser radiation. The bandwidth can be selected suchthat under all conditions at least one additional detector element ofthe spectral analyzer and direction indicator is illuminated with abroad source than is illuminated with a laser source. If the filter istoo wide, needless background radiation noise is introduced. Pulsecoding also could be included in the laser signal. Relative heading isobserved by noting the apparent position within the FOV of the centroidof both laser sources as a group. Range is computed by using theapparent angular separation of the sources. All of these observationscan be made directly with the disclosed spectral analyzer and directionindicator system.

FIG. 7 illustrates in plan view two pairs of lasers which radiaterespective beams into the flight path. Specifically, a forward pair oflasers 33L and 33R radiate respective beams 35L and 35R. The forwardlasers 33L and 33R are oriented substantially the same as the lasers 31Land 31R in FIG. 6. An aft pair of lasers 37L and 37R radiate respectivebeams 39L and 39R.

The use of the additional aft lasers 37L and 37R allows for thedetermination of the following additional information: (1) indication ofthe location of an aircraft in later stages of landing when it is beyondthe working region of the forward lasers 33L and 33R; and (2) indicationof the elevation approach angle of the aircraft relative to the runway.

The beam configuration of the aft lasers 37L and 37R is substantiallythe same as for the forward lasers 33L and 33R. However, different beampatterns and power levels may be appropriate for the aft lasers 37L and37R because of the shorter range over which they are utilized, thepossibly slower aircraft speed occurring when they are in view from theaircraft, and the more restricted volume of probable positions of theaircraft when the aft lasers 37L and 37R are in view. For improvedperformance, additional sets of lasers could be added at other points onthe runway. Indeed, many other laser patterns could be suggested thatare well within the general concept of this invention.

FIG. 8 shows the forward and aft lasers of FIG. 7 in an elevation view.The laser beams are shown radiating from towers above the level of therunway. The lasers are directed at different angles and have differentbeam widths. A typical instantaneous position of the aircraft isindicated by the symbol "X." The elevation approach angle relative tothe runway can be measured because, as can be concluded from FIG. 8, theapparent vertical separation of the two sets of sources can be observedfrom the aircraft. If the actual heading of the aircraft differssignificantly from the direction of its path, the on board spectralanalyzer and direction indicator can be rotated so as to bring the beampatterns within its FOV.

FIGS. 9a through 9e schematically illustrate graphic displays which maybe provided to the aircraft pilot on the basis of the informationdetected by the on board spectral analyzer and direction indicator.FIGS. 9a through 9e are based on a landing system having two pairs oflasers nominally as shown in FIGS. 7 and 8. Such graphic displays areintended to be representative of the scene as would be observed by thepilot and as provided by the spectral analyzer and direction indicator.

FIGS. 9a through 9e specifically illustrate a sequence of displaysindicative of the particular positions of an aircraft relative to therunway as it approaches the runway. FIG. 9a shows initial detection ofthe laser sources, and FIG. 9b shows detection of the separate forwardlasers. FIG. 9c shows detection of the aft lasers in addition to theforward lasers. FIG. 9d shows detection of all four lasers at shorterrange than for FIG. 9c. FIG. 9e shows detection of only the aft laserswhen the aircraft is beyond the forward lasers.

When all four lasers are detected and distinguishable (FIG. 9c and 9d)the graphical display shows a pattern of four spots. The lower pair ofspots on the display, which will always be more widely separated,correspond to the forward lasers, while the upper pair of spotscorrespond to the aft lasers.

The display of the four spots allows for determination of range andelevation angle. Range to the runway is computed by using the apparenthorizontal separation of the pairs of dots. Elevation angle with respectto the runway is indicated by the vertical separation between thematched pairs of dots. Azimuth and elevation with respect to theaircraft coordinate system are indicated respectively by the horizontaland vertical position of the centroid of the dot pattern. These patternsare produced directly from the output of the spectral analyzer anddirection indicator. While it is not basically an imaging device, thedisclosed spectral analyzer and direction indicator in combination withthe disclosed system of laser beams can generate information to producethis graphic display, which can be interpreted easily by the pilot andwhich is almost a pictorial image.

The effective range of such a landing system could extend out to severalkilometers with possibly only a few watts of output power from eachlaser, depending on conditions for optical transmission. One advantageof this system is that aircraft locations relative to the runway can beobtained with radio silence. This feature could be valuable for anaircraft landing system on a Navy aircraft carrier, for example. If along wavelength laser (such as CO₂) is used, then a degree of fogpenetration is obtained. Eye safety is an important consideration, andwhere there is a significant health risk, radiated power levels must bereduced appropriately, and/or wavelength must be restricted to aneye-safe spectral region. Typical windshield materials will blockradiation from CO₂ lasers. As an additional safety feature, lasers couldbe appropriately mounted on towers.

Another advantage of the disclosed landing system is that the pilot cancontrol the aircraft and guide it to a safe landing using reliableinformation available to him in the cockpit; he is not dependent onother individuals at some remote location. The laser installationproposed for the runway is neither particularly expensive to install norobtrusive to the normal operation of the runway. On the aircraft itself,the spectral disclosed analyzer and direction indicator is small andlight.

Although the foregoing has been a description and illustration ofspecific embodiments of the invention, various modifications and changesthereto can be made by persons skilled in the art without departing fromthe scope and spirit of the invention as defined by the followingclaims.

What is claimed is:
 1. An aircraft landing system comprising:a pluralityof laser sources in the proximity of a runway for radiating laserradiation indicative of the location of the runway, said laser radiationbeing radiated in a fixed non-scanning predetermined pattern with atleast two simultaneously radiating sources; and sensing means carried byan aircraft for detecting in staring fashion the spectral content andincidence directions of the laser radiation radiated by said lasersources to provide azimuth, elevation, and range information indicativeof aircraft position relative to the runway.
 2. The aircraft landingsystem of claim 1 wherein said sensing means comprises a spectralanalyzer and direction indicator.
 3. The aircraft landing system ofclaim 1 wherein said radiating means comprises plurality of lasersselectively positioned adjacent the runway.
 4. The aircraft landingsystem of claim 3 wherein said plurality of lasers includes a pair oflasers located on each side of the runway.
 5. The aircraft landingsystem of claim 3 wherein said plurality of lasers includes a first pairof lasers located on each side of the runway nearer to an approachingaircraft, and a second pair of lasers on each side of the runway furtherfrom the approaching aircraft.
 6. The aircraft landing system of claim 1wherein said sensing means includes display means for visuallydisplaying said azimuth, elevation and range information.